a) Field of the Invention
The present invention relates to a plate-like or sheet-like transparent member which is used to be disposed on a light emitting surface side of a surface light source device. Particularly, the present invention relates to a transparent member having a multiplicity of prism-like portions on one of surfaces thereof.
b) Description of the Prior Art
The transparent material according to the present invention is in a surface light source device, which is for use in a back light of liquid crystal display units. FIG. 1 is a schematic view showing a construction of a conventional surface light source device which has a light conducting member. In the surface light source device shown in FIG. 1, the numerical reference 5 represents a linear light source, 6 a light conducting member made of transparent material having a plate-like shape, 7 a diffusing plate, 8 represents a reflective member. In this surface light source device, a light emitted from the light source 5 is made incident upon an edge surface 6a of the light conducting member 6 and directed inside the member to be transmitted to the center portion thereof. A part of light being transmitted inside the light conducting member 6 is diffused by a multiplicity of fine diffusion portions 9 which are provided on a surface 6c of the light conducting member 6, and then emitted from the light emitting surface 6b thereof. The emitting light emitted from the light emitting surface 6b then passes through the diffusion plate 7 to become a diffusion light having an almost uniform brightness distribution for illuminating a liquid crystal display panel 4.
In this device, the diffusion light diffused by the diffusion plate 7 is directed in all directions. Therefore, there is some waste light which is not directed toward the liquid crystal display panel 4. Such waste light is not useful for illuminating the liquid crystal display panel 4 and thus a bright surface light source cannot be obtained.
In order to remove the drawback, there is arranged a transparent member 1 having a multiplicity of prism-like portions formed on one of the surfaces thereof between the diffusion plate 6 and the liquid crystal display panel 4 in the conventional surface light source device. By the transparent member 1, the light directed toward outside the liquid crystal display panel 4 is collected on the liquid crystal display panel 4 as much as possible to provide a bright surface light source.
The transparent member 1 has such a cross-sectional shape as shown in FIG. 3 that a multiplicity of triangles 10 are aligned to be formed on one of the surfaces thereof; and top lines of the triangle portions are arranged to be parallel to each other, as shown in a plan view of FIG. 2.
In FIG. 4, is shown another conventional transparent member, in which a multiplicity of conical or poligonal pyramid-shaped portions 104 are provided on one of the surfaces thereof in a crosswise manner. When disposing this transparent member between the surface light source device and the liquid crystal display panel as shown in FIG. 1, such trouble sometimes happens that a direction 1a along which the top lines of the triangle portions are extended or directions along which top portions of the conical or polgonal pyramid-shapes portions are aligned is or are lies upon bus lines of the liquid crystal display panel 4, and a stripe pattern as Moire, which is not desired, is generated.
Further, when the above-explained transparent member (shown in FIGS. 2 and 3) is disposed between the surface light source device and the liquid crystal display panel, most of the diffused light is directed in a perpendicular or almost perpendicular direction with respect to the liquid crystal display panel. Therefore, when observation is achieved from the direction being perpendicular with respect to the liquid crystal display panel, a desired bright image can be obtained, but when observation is from a perspective direction with respect to the liquid crystal display panel, the image becomes so dark that an observer can scarcely see the image.
In order to solve this drawback, still another conventional transparent member, which is as shown in FIG. 5, is suggested. In this member, the top portions 10a of the prisms 10B are arranged to be round; and most of the diffused light which has been passed through the diffusion plate 7 is directed toward the liquid crystal display panel 4 thereby to obtain a bright illumination. Since a part of the diffused light is proceeded in a perspective direction with respect to the liquid crystal display panel 4, the observation of the liquid crystal display panel from the perspective direction can be also carried out. However, there is still remained some light directed outside the liquid crystal display panel, which cannot be used for illuminating the panel; and then some loss of the amount of light is caused.